There is a long felt need to accurately measure the internal impedance of a battery unit. As used herein, a “battery unit” or “unit” is the smallest part of a battery that can be connected to a battery monitor. A battery unit may be, but is not limited to, one or more of:                Cell: An individual electrochemical device composed of two electrodes of dissimilar metals (active materials) and an electrolyte. When the electrodes are immersed in an electrolyte, the cell will produce a voltage differential between the electrodes. When connected to electrodes, the cell will produce a current through an external circuit.        Jar: One or more cells in a battery string manufactured and contained in a single container.        Monobloc: Two or more separate but electrically connected cells each of which is designed to house an assembly of electrodes, electrolyte, terminals or interconnections and possible separators. The cells in the monobloc can be connected in parallel or in series.        
As used herein, a “battery string” is one or more battery units connected in series to furnish an electrical current at a specified total terminal voltage. Multiple strings may be connected in parallel to attain a higher current or energy capacity in a battery system.
Battery units in a battery string may be connected by conductors known as “busbars”. As used herein, a bulbar can be any conductor in any form such as a solid bar, a cable or a flexible wire. Busbars may or may not be insulated.